1. Field of the Invention
The present invention is concerned with the separation of monovalent anions in a basic medium with respect to their relative basicity.
More especially this invention relates to a process that separates the liquors used in the kraft pulping process into two streams; one stream that is rich in alkali metal sulphide and another that is poor in alkali metal sulphide.
2. Description of Prior Art
The kraft pulping process is the most widely used to produce chemical pulps and as such has been the focus of much research and development. In the production of bleached kraft pulp, a low lignin content is desirable (both after pulping and after oxygen delignification), in order to decrease the consumption of expensive bleaching chemicals and to reduce the effect of bleaching effluents on the environment. Various modifications to the basic kraft pulping method have been suggested. The objective of these modifications has generally been to improve the selectivity of the pulping process. Improving selectivity in a pulping process means that the rate of delignification is high compared to the rate of cellulose and hemicellulose degradation. Two such processes are MCC--modified continuous cooking (B. Johansson et al., Svensk Papperstidning nr 10, 30-35, 1984) and EMCC--extended modified continuous cooking (J. E. Jiang et al., APPITA, 45 (1), 19, 1992). It is recognized that the selectivity of the process could be improved further by applying a high sodium sulphide to sodium hydroxide ratio at the beginning of the delignification phase (S. Norden and A. Teder, Tappi, 62( 7), 49-51, July 1979, A. Teder and L. Olm, Paperi ja Puu 63 (4a), 315-322, 1981, B. Mao and N. Hartler, Paperi ja Puu--Paper and Timber, Vol. 74, No. 6, 491-494, 1992).
In a continuous digester system using MCC or EMCC, it is standard practice to divide the total effective alkali (EA) charge into three or four portions. It is simultaneous control of this EA distribution and the dissolved lignin profile that makes selective extended delignification possible. Dividing the total EA addition into three charges results in a slower pulping rate; this effect can be minimized by adding all of the sodium sulphide to the first EA charge in the impregnation stage. This improvement in pulping seems to be additive to that of modified and extended modified cooking (J. E. Jiang et al., CTAPI, Proceedings of 7th Int. Symp. on Wood and Pulping Chemistry, Vol. 1, 337-347, Beijing, P. R. China, May 25-28, 1993). A low concentration of dissolved lignin and a high alkali concentration in the final delignification phase increases the lignin removal (K. Sjoblom et al., Paperi ja Puu--Paper and Timber, 5, 452-460, 1988, B. Mao and N. Hartler, Nordic Pulp and Paper Research Journal, No. 4, 168-173, 1992). A process that generates a sulphide-rich and a sulphide-poor liquor while maintaining the sulphur balance in the kraft process would help in improving the pulping process.
Extended modified cooking has also been examined in the presence of polysulphide which increases pulp yield. Extended modified cooking and polysulphide pulping are two compatible processes that offer complementary advantages (J. E. Jiang, Tappi, 77(2), 120-124, February 1994). A process that provides a high polysulphide concentration in the presence of a decreased concentration of hydroxide would allow further improvements of the pulping process.
At present, there are three published methods of producing liquors with different sulphidities, although none has been tried on a commercial scale. H. A. Simons Ltd. (P. P. H. Lownertz, World patent, WO92/20856, 26 Nov. 1992) proposed a process in which the sodium sulphide content of recovery boiler smelt is leached with water or sulphide-poor white liquor. The solid sodium carbonate content is separated from the sulphide-rich green liquor and dissolved prior to causticizing. The sodium carbonate solution is causticized to a sulphide-poor white liquor. This sulphide-poor liquor can be used for leaching the smelt, for oxygen delignification and for flue gas scrubbing. The sulphide-rich white liquor can be used in the initial phase as well as at the beginning of the bulk delignification phase of the pulping process. The system used is based on technology developed for neutral sulphite semichemical (NSSC) recovery (S. Mizuguichi and T. Naito, Pulp & Paper Canada, 79(8), T251-253, 1978), but is modified to avoid oxidation of sulphide, and to utilize the heat content of the smelt to evaporate water from the liquor.
Another system proposed by Ahlstrom Corporation is a combination of liquor heat treatment (R. Ryham and S. Nikkanen, Proceedings of the 4th SPCI International Conference: Book 1, pp.266-280, Bologna, Italy, May 19-22, 1992) and the DESULPHUR process (R. Ryham and H. Lindberg, 80th Annual Meeting, CPPA Technical Section, B179-B190, February 1994). The basic step in the DESULPHUR process is heat treatment of black liquor, in a process where up to 50% of the sulphur content in the black liquor is released as methyl mercaptan and dimethyl sulphide. The organic sulphides are then converted to hydrogen sulphide. Scrubbing the hydrogen sulphide gas with white liquor provides a white liquor with high sulphidity. In this way about 75% of the total sulphide charge is concentrated in a stream containing about 60% of the total charge of effective alkali. This composition approximates the chemical requirement in the impregnation stage of modified cooking processes.
White liquor with split sulphidity can also be prepared by crystallizing sodium carbonate from well-purified green liquor, Green Liquor Cooling Crystallization (GLCC), (R. Ryham and H. Lindberg, 80th Annual Meeting, CPPA Technical Section, B179-B190, February 1994). The crystallization is accomplished by simultaneous evaporation and cooling of green liquor to about 12.degree. C. The alkali metal carbonate crystals are separated and dissolved prior to causticizing. In this way it is possible to produce a stream that contains about 60% of the EA charged to the digester. The remaining 40% of the EA will be in a sulphur-free caustic stream. Compared to a normal white liquor having 38% sulphidity, the sulphide-rich white liquor would have a sulphidity of about 62%.
Since white liquor is generated in a largely closed causticizing cycle, the ratio of Na.sub.2 S to NaOH remains constant. In order to obtain two streams of white liquor with different concentrations of Na.sub.2 S and NaOH, while maintaining the overall sulphidity in the recovery cycle, electrodialysis of white liquor prior to cooking is proposed.
Electrodialytic systems have been generally used for the separation and concentration of electrolytes (H. Hirai, S. Matsushita and I. Tsuyuki, U.S. Pat. No. 4,207,157, Jun. 10, 1980, D. J. Vaughan, U.S. Pat. No. 4,325,792, Apr. 20, 1982). More particularly, they have been used to separate monovalent anions and cations from multivalent anions and cations by incorporating into these systems monovalent anion- and cation-selective membranes. Examples of these applications include the separation of chloride and fluoride ions from a metal sulphate solution (D. L. Ball and D. A. D. Boateng, U.S. Pat. No. 4,715,939, Dec. 29, 1987), the separation of chloride ions from sulphate ions in seawater (G. Saracco and M. C. Zanetti, Ind. Eng. Chem. Res., 33, 96-101, 1994) and the separation of chloride ions from oxalate ions in an industrial waste water from electrolytic production of titanium (G. Saracco, M. C. Zanetti and M. Onofrio, Ind. Eng. Chem. Res., 32, 675-662, 1993). In a recent patent (M. W. Kennedy et al., U.S. Pat. No. 5,324,403, Jun. 28, 1994), electrodialysis is used to separate sulphate and thiosulphate from a hydrogen sulphide scrubber solution of the liquid redox type. There are, however, no published data on the selectivity of anion-selective membranes towards monovalent anions on the basis of their basicity, nor are there any published methods of separating hydrosulphide from kraft mill white liquor (a mixture of sodium sulphide and sodium hydroxide) and from kraft mill green liquor (a mixture of sodium sulphide, sodium carbonate and sodium hydroxide) by electrodialysis.
There are also, no published data on the separation of hydroxide from polysulphide in a Kraft mill polysulphide liquor.